The prevalence of low bone mineral density (BMD) is high among young patients with inflammatory bowel disease (IBD), during a critical period for bone mass acquisition. Guidelines for the treatment or prevention of this condition in children are lacking. Hypovitaminosis D is also prevalent in this population, and there are no guidelines for its treatment. Furthermore, the Candidate has obtained evidence that PDA recommended doses of vitamin D supplementation are inadequate to maintain optimal vitamin D stores throughout the year. Vitamin D may be important for bone health in states of inflammation, through its anabolic effects on bone, and its regulatory effect on the immune system. Further studies are needed to establish guidelines for the treatment and prevention of hypovitaminosis D in children with IBD, and examine the effects of these interventions on bone health. This Award will permit the Candidate to prepare for a career as an investigator of bone health in children with chronic disease, starting by filling these gaps in the care of children with IBD. With the mentorship of experts in her team, she plans to conduct 2 clinical trials. In the first, she will compare the efficacy of 3 vitamin D regimens for the treatment of hypovitaminosis D in children with IBD. In the second, she will compare the efficacy of 2 vitamin D supplementation regimens for the maintenance of optimal vitamin D stores throughout the year, and their effects on the bone health of these subjects over a period of 2 years. Bone health will be assessed measuring: a) BMD using dual energy x-ray absorptiometry (DXA) and peripheral quantitative computerized tomography (pQCT), and b) bone strength using pQCT. The short term goals of this proposal are to allow the Candidate to acquire in depth knowledge on the subject of bone health and metabolism and their relationship to disease, learn specific biostatistical and methodological skills to guide her in the conduct of research in these fields, and receive valuable mentorship to help her evolve into an independent clinical investigator. The long term goals of this project are to establish monitoring of vitamin D status as a standard of care for children with IBD, create guidelines for the treatment and prevention of hypovitaminosis D in this population, and observe the effect of these interventions on bone health. The impact of the results of this research on other pediatric populations with diseases that affect their bone health could be great.